This invention relates to a method for preparing high purity vanadium metal. More specifically, this invention relates to a method of preparing high purity vanadium metal which is relatively free of silicon from commercial grade vanadium pentoxide.
There is a great deal of interest in vanadium for use in several important high technology applications. Vanadium-based-alloys have a low capture cross-section for high energy neutrons and are resistant to void swelling, and so are being considered as a candidate material for cladding and ducts in fast breeder reactors and as a first wall in fusion reactors. Another use for vanadium is the super-conducting A15 compound, V.sub.3 Ga which has a higher critical current density (J.sub.c) than Nb.sub.3 Sn in magnetic field intensities larger than 6 tesla, in the temperature range of 4.2 to 10 K. However, investigations have indicated that the presence of silicon may have a detrimental effect on the superconducting and fabrication properties of vanadium alloys.
Efforts to purify vanadium of silicon have proven to be difficult and/or expensive. For example silicon in vanadium cannot be removed by electrotransport purification due to its relative immobility in the vanadium matrix. Nor can silicon be preferentially evaporated from vanadium during electron beam float zone melting (EBFZM) even though the relative vapor pressures of vanadium and silicon would indicate this might occur.
Most commercial grades of vanadium, metal contain between 200 and 800 ppmw parts per million weight silicon in addition to small amounts of aluminum, carbon, iron and molybdenum. Commercial grade V.sub.2 O.sub.5 generally contains 300-400 ppm silicon which carries over to the vanadium metal upon reduction. At present, reduction of silicon in vanadium metal can be accomplished by either reducing the silicon content in the V.sub.2 O.sub.5 before reduction to the metal or by removing silicon from the already reduced metal.
Methods for reducing silicon in the pentoxide include: fusing the vanadium pentoxide with ammonium bifluoride. Upon dissolution in water followed by reprecipitation as ammonium metavanadate, an oxide product is produced in which the silicon content is decreased from 250 ppmw to approximately 75 ppmw. Another method involves an ion exchange separation and consists of absorbing VO.sup.++ and Fe.sup.3 + ions on an ion exchange column while the Si.sup.4+ ions pass through. The vanadium and iron ions on the column are then separated by complexing with ethylenediaminetetraacetic acid (EDTA) and the vanadium is recovered as the purified pentoxide containing less than 10 ppmw of both silicon and iron.
Silicon can be removed from vanadium metal by an iodine refining process in which the silicon content is reduced from 300 ppmw to about 50 ppmw. A fused salt electrorefining process was found capable of removing virtually all of the silicon from vanadium. In this process feed material containing 4200 ppmw Si was purified into metal containing about 60 ppmw Si in a single refining step using a LiCl-KCl-VCl.sub.2 electrolyte. By a double electrorefining step, silicon content was further reduced to about 15 ppmw.
Quantities of commercial grade, low-oxygen vanadium metal are produced by the aluminothermic reduction of commercial grade V.sub.2 O.sub.5 in a water-cooled crucible, as described in J. Metals, 18 (3) (1966), pp. 320-323. The essential feature of the process is the addition of an excess of aluminum metal to form a vanadium-11% aluminum alloy containing about 0.5% oxygen as the reduction product. The excess aluminum is present in the alloy so that upon subsequent heating in vacuum at high temperatures, the aluminum is vaporized, simultaneously removing the residual oxygen as the volatile suboxide, Al.sub.2 O. The resulting sponge product is then electron-beam melted to yield metal of 99.9+% purity containing about 50 ppmw oxygen. The product also contains about 500 ppmw silicon, which is virtually the same silicon that was in the V.sub.2 O.sub.5 starting material, since the reaction process removes little or no silicon.
All of the above methods which remove silicon from either V.sub.2 O.sub.5 or vanadium metal are complex and hence expensive processes which greatly increase the cost of vanadium, while the usual method for the reduction of V.sub.2 O.sub.5 removes little or none of the silicon which was present in the starting material. What is needed is a relatively inexpensive process which will reduce commercial grade vanadium pentoxide to the metal while decreasing the silicon content at the same time.